Q: my iphone is attached to a itunes user on a dead mac and will not sync to my new imac itunes?

What error message are you getting on the new iMac when you try to sync the phone?
Also you can move your iTunes library from the old hard drive to the new one

If you had more than one computer authorized you can use "deauthorize all computers" to deauthorize the dead one.

See here. Deauthorize your computer using iTunes - Apple Support

However, there are complications with dissociating your iOS devices from your Apple ID. Something about a 90-day wait.

Probably best to talk to Apple Support.http://www.apple.com/support/appleid/

im told this account is on the macbook pro that has died.

The account is with the iTunes Store, and the dead computer was authorized to access that account.

BTW, did or do you have more than one Apple ID?